[Exim] Empty lookup result in authenticators (a stupid quest…

        Hello!

    I'm using exim 4.12 .
    I configured authenticators like:

---------------------------------------
begin authenticators

login:
    driver = plaintext
    public_name = LOGIN
    server_prompts = Username:: : Password::
    hide server_condition = ${if eq{$2}{PASSWORD_LOOKUP}{yes}{no}}
    server_set_id = $1
---------------------------------------
    If PASSWORD_LOOKUP doesn't end with "fail", it accepts
empty passwords for unknown accounts.
    If PASSWORD_LOOKUP ends with "fail" I get the following
reply from exim in SMTP session:

--------------------------------------
AUTH login
334 VXNlcm5hbWU6
aHJicmZy
334 UGFzc3dvcmQ6

435 Unable to authenticate at present
--------------------------------------
    But how could I get the proper "535 Incorrect authentication
data" ? If I check for empty passwords within server_condition, I deny
such passwords at all. It seems to be not very good.

    Any ideas? Is there  a way to distinguish between "not
found" lookup result, and "lookup failed" (server error for example)?

        Bye. Alex.